Such position-measuring devices are used, in particular, in machine tools for measuring the position of a tool relative to a workpiece to be machined, in coordinate measuring machines for determining the position and dimensions of test objects, as well as in the semiconductor industry, for example, in wafer steppers and bonders. In such devices, the scale is directly attached to the drive unit (e.g., a linear motor), or the scale is attached to a component driven by the drive unit. The scanning unit of the position-measuring device is stationarily disposed opposite the moving scale on another machine part whose position is to be measured.
A position-measuring device of this type is known, for example, from U.S. 7,421,800 B2. The scale has an incremental measuring graduation for generating periodic measurement signals, as well as a plurality of reference marks spaced apart in the measurement direction and used for generating reference mark signals. The function of a reference mark signal is to establish an absolute reference of the incremental position measurement for the position of the reference mark by setting a counter to a predetermined counter value. In order to make this position selectable for the user, the user may select from the plurality of reference marks. For selection purposes, a first marking in a marking track of the scale is associated with the desired reference mark and generates a selection signal when scanned. Also located in this marking track is a second marking from which a switching signal defining a limit position can be produced by scanning.
In order to distinguish the two markings located at different positions in the marking track, a further marking is located at the position of the second marking in a further marking track. To increase the reliability of discrimination, it is proposed to make the second marking longer than the further marking located in the further marking track.